We got a similar surprise last year. We reserved two nights at Steamboat Rock State park in Eastern Washington, not realising how hot it was going to get. After the first night we decided to leave, since our generator was giving us some grief and we needed both a/cs.
The office was closed, so I left a note offering the balance of my fee to whoever got the site. About 10 days later, the refund showed up.
Frank Damp, DW - Eileen, pet - female Labrador (10 yrs old), location Anacortes, WA, retired RVers (since Dec 2014)